In recent years, various technologies have been developed from a viewpoint of reducing the quantity of fuel consumed in a vehicle. For example, there have been developed a hybrid vehicle, which has an electric motor as a main unit mounted in a vehicle in addition to an engine, and an idling stop system for automatically stopping an engine when a vehicle stops.
In general, in the heating of the interior of a cabin, heat wasted to the coolant and the rest from an engine is utilized. However, in the low fuel consumption vehicle taken as an example, the heat power wasted from the engine is reduced because of an idling stop and an improvement in the efficiency of the engine itself, which causes a possibility that a heat power necessary for heating cannot be ensured only by the waste heat from the engine.
Thus, there is proposed a construction in which a vehicle has a heating device of a heat pump type, which is driven by an electric motor, in addition to a heating device utilizing the waste heat from the engine (see, for example, Japanese Patent Publication No. 3704788).
By the way, the construction described in Japanese Patent Publication No. 3704788 includes a plurality of heat sources used for heating the interior of the cabin and hence presents a problem how much and which of the heat sources is to be used from the viewpoint of efficient use of energy.
However, the construction described in Japanese Patent Publication No. 3704788 does not establish a principle of supplying heat from the plurality of heat sources but still has plenty room for improvement. Moreover, in the case where an abnormal heat source which cannot properly supply heat is caused in the plurality of heat sources, how to supply the heat from the plurality of heat sources including the abnormal heat source becomes a problem.